Silver is recognized as a most preferable material as a material for reflective films to be used in optical recording media, displays and the like. This is because silver is high in reflectance, and additionally, lower in price than gold that is also high in reflectance. In the field of optical recording media in these years, transition to recordable/rewritable media (CD-R/RW, DVD-R/RW/RAM) takes place, and expanding importance of optical recording media attaches importance to silver that is high in reflectance and relatively low in cost as a central material for use in reflective films.
On the other hand, silver involves a problem such that silver is poor in corrosion resistance and is degraded in reflectance by discoloration to black through corrosion. The root causes for the corrosion of reflective films may vary with products (recording media, displays and the like) to which the reflective films are applied, in such a way that, for example, reflective films of displays suffer from a fear that they are corroded due to the atmospheric humidity or the like.
A reflective layer of an optical recording medium also suffers from a fear of corrosion due to the atmosphere such as the air, and furthermore, requires a consideration of corrosion caused by the effects of the other constituent layers (the recording layer, dielectric layer and the like) in contact with the reflective layer in the recording medium. For example, a recordable optical disk (CD-R, DVD-R, DVD+R or the like) has a structure in which an organic azo or cyanine dye ink is coated on a polycarbonate substrate and dried to form a recording layer, a reflective layer is formed on the surface of the recording layer, and the recording layer with the reflective layer is bonded to another polycarbonate substrate with an ultraviolet cure adhesive. In this case, the organic dye ink in the recording layer and the ultraviolet cure adhesive, both in contact with the reflective layer, contain sulfur as a component thereof or as an impurity, and accordingly, the reflective layer is exposed to a fear of corrosion in the course of use thereof because silver is poor in resistance to sulfidation. On the other hand, a rewritable optical disk (CD-RW, DVD-RW, DVD+RW, DVD-RAM or the like) has a structure in which a derivative layer, a recording layer, a dielectric layer and a reflective layer are laminated in such a condition that the reflective layer is in contact with the dielectric layer. Various materials are available for the dielectric layer, but in these years, materials to which zinc sulfide (ZnS) is applied are being used (sometimes, for the purpose of controlling heat reserve, a material made of zinc sulfide doped with silicon oxide, namely, ZnS-20 mol % SiO2 or the like being applied). Consequently, also in this case, there is a fear of corrosion due to a sulfide. As described above, the reflective layer of an optical recording medium is in such an environment that the reflective layer is in contact with a layer containing sulfur or a sulfide, irrespective as to whether the optical recording medium is recordable or rewritable in type; thus, the resistance to sulfidation comes to be more significant than the corrosion caused by the atmosphere involving humidity or the like, and there is a fear that the reflectance of the reflective layer is degraded by the long-term use of the recording medium.
Additionally, there is a problem that a reflective film made of silver is also thermally degraded in reflectance. The mechanism involved in the reflectance degradation due to heating is not yet elucidated, but it has been verified that heating of a thin silver film causes a phenomenon in which local agglomeration occurs in the thin film so as to expose the underlayer. Accordingly, the reflective film of an optical recording medium, a plasma display or the like is required to have a heat resistance because it possibly undergoes heating.
For the purpose of coping with the reflectance degradation of the reflective film as described above, there have conventionally been developed silver alloys, for use in reflective films, that are improved in corrosion resistance and heat resistance while the reflectance being secured. These alloys mostly include silver as the main component, and are doped with one or more of various elements as dopant elements; examples of such disclosed alloys include, for example, an alloy in which silver is doped with 0.5 to 10 atomic % of ruthenium and 0.1 to 10 atomic % of aluminum, and another alloy in which silver is doped with 0.5 to 4.9 atomic % of palladium. It has been disclosed that these silver alloys are satisfactory in corrosion resistance, can maintain the reflectance in service environment, and consequently, suitable for reflective films (for the details of the conventional art, see Patent Documents 1 and 2).
Patent Document 1: Japanese Patent Laid-Open No. Hei 11-134715
Patent Document 2: Japanese Patent Laid-Open No. 2000-109943
As for the above described silver alloys, improvements of the corrosion resistance and heat resistance have been developed to some extents. However, even these silver alloys are not absolutely free from deterioration in service environment. Additionally, the reflectance is not completely guaranteed against reflectance degradation, and materials that can maintain the reflectance at a higher level are desired.
In the field of optical recording devices, at present, red semiconductor lasers (wavelength: 650 nm) are applied as recording light sources, but blue lasers (wavelength: 405 nm) have almost seen their way clear to practical use. Application of the blue laser can ensure a memory capacity 5 to 6 times as large as the memory capacity of an optical recording device available at present, so that optical recording devices incorporating blue lasers applied thereto will conceivably form the mainstream of the next-generation optical recording devices. In this connection, according to the present inventors, it has been verified that the reflectance of a reflective film varies with the wavelength of the incident laser light; in particular, it has been verified that shorter-wavelength laser irradiation degrades the reflectance irrespective as to whether corrosion occurs or not, and makes the extent of the reflectance degradation due to corrosion frequently larger than the longer-wavelength laser irradiation. Consequently, for the purpose of producing recording media adaptable to the development of the future recording light sources, it is desired to develop a material that has a high reflectance even for the laser irradiation in a shorter wavelength region, and furthermore, can maintain the reflectance within a range of practical use.
The present invention has been achieved on the above described background, and is aimed at providing a material that is a silver alloy to form a reflective film of an optical recording medium or the like, and is workable without degrading the reflectance even in a long-term use. Additionally, the present invention provides a material that has a high reflectance even for a short wavelength laser light.